I. Field of the Invention
The invention relates to a sealing means for a casing of an electronic component. Preferably, the invention concerns sealing means for a capacitor tubular case.
The invention furthermore relates to a method for manufacturing such a sealing means.
II. Description of the Prior Art
It is known to seal tubular cases for electric capacitors either by elastomer plugs (German Pat. No. 23 09 492 Al) or by disks (German Pat. Nos. 23 07 068 Al; 27 18 228 Al). Regarding tubular cases of small diameters, the more economical and simpler, more reliable and better sealing plugs are used, while tubular cases of large diameters require the more expensive and as concerns sealing more critical disks, which on the other hand are more stable dimensionally.
Because of their relative large elastomer mass, the elastomer plugs offer a non-critical, good high deformability, that is, they provide high return forces and, therefore, will seal reliably. As a rule the tubular case is sealed by rolled-in pleats and/or by rimming and bending, and by the deformations in the elastomer plug so caused.
The relatively high permeability of elastomers with respect to liquids sealed in the tubular case is critical in the known plugs, these liquids being in particular electrolytes and dielectrics. In the early days of the plug technology, the plug material used were elastomers crosslinked with sulfur. It was found, however, that such sulfur-crosslinked elastomers in the course of time discharged sulfur into the capacitor liquid, whereby capacitor life ended prematurely. As a result, peroxidically crosslinking elastomers, especially EPDM, have been used as plug material. In the meantime it was found, however, that the permeability, especially of EPDM, is inadmissibly high.
Even for relatively thick plug designs, the liquid sealed in the tubular case in time will permeate the plug material and reach its outer surface. The liquid discharging at the outer surface evaporates, whereby constant evaporation of the liquid sealed in the tubular case takes place through the plug. This permeability is so high that modern sealing technology has gone back to plug materials which can crosslink, not with peroxides but only with sulfur. In particular, butyl rubber is being increasingly used as a material for elastomer plugs. However, while butyl rubber evinces only a relatively low permeability, though larger than null, it also suffers from the substantial drawback that it is difficult to work. Furthermore, butyl rubber cannot be peroxidically crosslinked, rather it must be subjected to a chemically undesired sulfur crosslinking, for instance with mercaptans or thiurams, or to process-technically disadvantageous resin crosslinking, for instance with coumarone resins.
Both the high permeability of EPDM and the difficult workability, plus any possible sulfur content of butyl rubber, prevent using elastomer plugs to seal tubular cases of substantial diameters. Furthermore, problems of dimensional stability prevent using such plugs on merely mechanical considerations.
Accordingly, dimensionally stable disks made from rigid materials such as plastics, molded substances or metals are used to seal large-diameter tubular cases. Particularly as regards economical manufacture and good temperature resistance, metal sealing disks, especially made of aluminum, have been found practical in recent times. The gas-tight and liquid-tight sealing between such aluminum disks and the rim of the tubular case, also consisting most of the time of aluminum, is implemented by an elastomer which most of the time also is used for electrically insulating the metal sealing disk from the tubular case. The elastomer is designed in the form of a relatively thin foil which is bonded, vulcanized or riveted to the inside of the metal sealing disk that faces toward the inside of the tubular case. The diameter of the circular elastomer foil exceeds the diameter of the metal disk, so that the elastomer disk--when the sealing disk is inserted into the aperture of the tubular case--bends over and around the metal disk onto its outside by its projecting rim area. In the ensuing deformation of the rim of the tubular case, the elastomer foil between the rim of the tubular case and the rim of the metal disk is deformed so as to seal (German Pat. No. 23 07 068 Al). The electrical feedthroughs are designed in the manner of rivets, the one head of which rests on the inside of the elastomer foil and the other head on the outside of the metal disk.
In this known method of disk sealing using metal disks, there is a drawback in that the relatively thin elastomer foil can lead to leakages either by being pleated or by being torn when the capacitor is being sealed. If the elastomer foil is made thicker so as to prevent tearing, there is an increased tendency for material buckling, thereby forming canals which prevent a possible tight sealing of the tubular case. If the foil is made thinner to prevent canal formation, there is again the tendency to tear. Useful results by using intermediary values for foil thickness can be obtained, but they are not wholly satisfactory in all respects.
A better solution for sealing between the metal disk and the tubular case of the capacitor is the sealing disk known from German Pat. No. 27 18 228 Al. As regards this sealing disk, the rubber sealing element is injectionextrusion coated on the upper, lower and radial outer sides of the metal disk rim. This permits reinforcing the rim-rubberizing of the metal disk without incurring stress. This reinforcement, however, is restricted, and in the course of sealing the tubular case and radially pressing against the tubular case wall, the rubber may be detached from the metal disk and, hence, leakage channels may be generated. Also, the rim-rubberized disk cannot be equipped with insulated electric feedthroughs.
In view of this state of the art, it is the object of the present invention to create a sealing means of the initially cited kind which shall be impermeable, dimensionally stable and problem-free in engineering, as these terms are explained above, and which simultaneously shall behave reliably and be noncritically sealing when the tubular case is being closed.
Additional objects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.